Agreed on the whole "Don't throw the word 'love' around" thing, but beyond that, relationships change. What started out as mono can turn into poly, and vice-versa.
One thing is for sure though, whether you're poly or not, you're not going to be happy unless you can share your feelings with your partner. That'll eat you away inside, and you'll wind up drunk and singing "Rocket Man" at a local Karaoke night before going home with some skeezy drunk who you'll get gonorrhea from and regret sleeping with forever. ....ok, well, maybe not all of that (at least, I sure hope not), but it'll definitely eat you up inside and make you either unhappy or numb, neither of which are any good at all. Thi is going to sound cliché, but communication is the bedrock upon which to build a stable relationship. I'm not sure how best to bring up the subject with your lady, perhaps discuss what's happened in the past, and talk about how you felt about her during that time... I dunno, but I think it needs to be done, even if you guys remain mono while you work on things. You don't have to dive in, but it's certainly important to you, therefore I think it must be discussed; even if the end result is not what you hoped for, at least you'll be able to communicate with your lover, and that's of tantamount importance.
What makes you think she could never go for a poly relationship? If she already accepted you for who you are, in spite of you having fallen in love with her best friend, perhaps she's more accepting than you think. Maybe she's had similar feelings about multiple men in the past. Like I said, talk to her about it. ....and good luck amigo.